1. Field of the Invention
The present invention relates to disk drives used in computer systems. More particularly, the present invention relates to a disk drive using loopback to calibrate transmission amplitude.
2. Description of the Prior Art
FIG. 1 shows a typical configuration of a serial attached host 2 connected to a serial attached disk drive 4, such as a serial advanced technology attachment (SATA) disk drive. The serial attached disk drive 4 comprises one or more disks 3 with one or more heads 5 for accessing respective disk surfaces. The heads 5 are connected to the distal end of actuator arms which are rotated about a pivot by a voice coil motor (VCM) 7 in order to actuate the heads 5 radially over the disk surfaces. The disks 3, heads 5, and VCM 7 are enclosed in a head disk assembly (HDA) 9 and controlled by control circuitry mounted on a printed circuit board coupled to the HDA 9. The control circuitry includes interface circuitry 11 for interfacing with the serial attached host 2 over a serial connection.
The interface circuitry 11 in both the serial attached host 2 and serial attached disk drive 4 of FIG. 1 implements an application layer 6, a link/transport layer 8, and a physical layer 10. The application layer 6 includes application and driver software that create specific tasks for the transport layer 8 to process. The transport layer 8 encapsulates commands, data, status, etc., from the application layer 6 into frames to be transmitted, and receives frames, disassembles the frames, and sends the frames' content to the application layer 6. The transport layer 8 sends messages to the link layer in order to establish connections. The link layer 8 controls the physical layer 10 for connection management. The physical layer 10 contains the hardware (such as transceivers and the encoding machines) that connects to the physical interface and sends the signals out on the wire.
The specification for the electrical interface, including the signaling amplitude, may vary depending on the types of serial attached disk drive. For example, the SATA interface specifies a transmission voltage in the range of 400–600 mV and a reception voltage in the range of 325–600 mV, whereas the serial attached SCSI (SAS) interface specifies a transmission voltage in the range of 800–1600 mV and a reception voltage in the range of 275–1600 mV. To be considered compliant, a serial attached disk drive should operate within the limits of its particular interface specification.
In some cases, it may be desirable for a serial attached disk drive to operate with a number of different interface standards. For example, it may be desirable for a SATA disk drive to be capable of connecting to either a SATA host or a SAS host. However, the signaling amplitude of the drive's SATA interface may be insufficient to communicate reliably with a SAS host, particularly if the communication channel attenuates the signaling. For example, the length of the serial communication cable contributes to signal attenuation, particularly for external disk drives connected to a host through a long cable. Channel attenuation may also decrease reliability for compatible devices, for example, if a SATA disk drive is connected to a SATA host through a long cable. In addition, the signaling amplitude selected by a SATA disk drive impacts power consumption, which is of particular concern in portable applications.
There is, therefore, a need to optimize the signaling amplitude in a disk drive so that, for example, the disk drive may operate with a number of different interface standards while remaining compliant, compensate for channel attenuation, and/or reduce power consumption in portable applications.